Design proposal: general 'design your own format' engine (old-school/EC retro formats as first presets)

[rykerwilliams]

Hi! Thank you for building this — the rules engine and UI are genuinely impressive.

I put together a design research pass (AI-assisted, disclosed) for a general "design your own format" capability, motivated by wanting to support retro/old-school constructed formats — specifically Eternal Central's four community formats: Old School 93-94, Old School 95, Middle School, and Classic Magic.

Rather than proposing to hardcode those four, the design centers on a general, data-driven layer:

• A GameFormat::Custom variant + CustomFormatDef schema (legal set list, banned/restricted names, deck-size/sideboard rules) — new formats become data, not a code change.
• An independently-toggleable LegacyRuleSet for pre-modern rules that don't all travel together — e.g. mana burn applies to all four EC formats, but "damage uses the stack" and pre-M10 Wish-to-exile access only apply to two of them, so they can't be a single "old rules" flag.
• The four EC formats (plus a Lords-of-the-Pit "Eternal Chaos" variant as a stretch goal) as validating presets built on top of the general schema, proving it works rather than being bespoke implementations.
• Cross-checked against New Format Plan: Dandan #5169 (the Dandan shared-library format plan) to make sure these two efforts don't collide — they're complementary rather than competing: Dandan needs genuinely new engine behavior (shared zones, a new dealer), which a bespoke GameFormat variant is the right tool for, while this proposal is purely a configuration layer over existing rules.

This is a discussion artifact, not a code PR — full research + design docs are here: rykerwilliams#1 (opened against a mirror of main just to get a readable diff, not meant to merge as-is).

Before anyone puts implementation time into this, I'd love feedback on whether the overall direction is right, and specifically whether a player-facing "format builder" UI, an operator/deployment-level config file, or both sharing one schema is the delivery shape you'd want.